A new 90-line Mirror Screw

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A new 90-line Mirror Screw

Postby Panrock » Sat Oct 05, 2024 2:05 am

Hi. I'm back again after several years. Well, you can't keep a good hobby down! Mods, please move this thread to where you deem appropriate, if necessary. Thanks.

As you'll have probably gathered from a previous thread on this board, my last major project was a stainless-steel 120-line colour Mirror Screw. In this, I received help from our very own Steve_A with the electronics and the much-missed Karen Orton, with the timebase correction.

However, this was a real brute and it proved impracticably heavy to take to conventions, along with its bulky special supporting bench. It also required a lot of clear floor space to enable viewing at the correct aspect ratio - and provide a buffer zone to give a greater feeling of safety from the jagged 44lb mass whirling at 25Hz!

So, I have decided to make a much smaller and lighter Mirror Screw, from aluminium (not stainless steel), again with colour but this time using 90-line definition. Much of the existing 120-line supporting gear can be re-used. The unit will be suitable to place on a table, and will give a picture about 5 inches across. It will run on the 1932 TeKaDe 90-line standard.

It would be nice to house this in an attractive 'period' cabinet too.

I have the benefit of some suitable parts lying around, left over from previous projects. For example, this motor and bearing/shaft set (from a long-gone mechanical colour camera) could find new life in the Mirror Screw.

I will post a blow-by-blow update here as the project (hopefully) progresses.

Steve O
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Re: A new 90-line Mirror Screw

Postby Panrock » Sat Oct 05, 2024 4:16 am

Here's a diagram showing how the mirror screw 'slats' will sit on top of each other. One of the slats is coloured yellow here, to make the idea clearer. Only 8 of the 90 slats are shown. Each is rotated 4 degrees (for 90-lines) with respect to its neighbour.

A overlap of four slats at the ends will give sufficient of rigidity of the whole stack, traded off against overall weight. This means the slats will each measure 135 x 18 mm - to give a 4:3 picture with guard bands, made from 1mm thick aluminium stock, which determines the line 'thickness'. (One of the nice things about a well set up mirror screw is that there is no obvious line structure visible.)

The board software seems to be re-sizing my images. If you can tell me what the native pixel counts are, I can work to those figures and improve the quality.

Steve O
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Re: A new 90-line Mirror Screw

Postby Steve Anderson » Sat Oct 05, 2024 3:31 pm

Good to see you back here Steve! I doubt that the GIF image would be used for the production of the slats, so it's more down to the precision of the software creating the file that the CNC machine/laser cutter (or whatever) will use as well as the precision mechanically of that machine. The image posted has 'jagged edges', such is the nature of raster-based images. It's certainly good enough to illustrate the concept on-screen. It also depends on the resolution and settings of each individual PC display used.

Each slat has only has 5 critical points, each corner and the centre shaft position. That's all that's needed.

Later how to increment the rotational position of each slat accuratly before clamping/locking the whole assemby together as one lump needs thought...but you've done this before, so it should a breeze with the use of Karen Orton's timebase corrector, though that might (probably will) need modification due to the different line quantity.

Steve A.
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Re: A new 90-line Mirror Screw

Postby Panrock » Sat Oct 05, 2024 10:47 pm

Hi Steve, thanks for the welcome.

Yes, the .gif image was only to illustrate how the slats sit on one other... nothing more.

I had hoped I could order the slats pre-cut from my usual aluminium stockist... they do a lovely clean edge cut... but sadly I now see the dimensions lie outside their permitted range.

So it now looks like I will be producing a CAD file to take to the laser cutters. I might be simply able to re-proportion the original 120-line pattern, which is presumably still sitting on an ancient PC in the workshop. Fortunately, following the replacement of several electrolytics on the motherboard, this computer still works!

Another problem: I see that my local cutters, K & R Laser Services, are now 'permanently closed'. Time marches on.

I had hoped that by using aluminium this time, the slat edges would be much easier to polish than with stainless steel. With the 120-line rig, I had to have the hard stainless steel slats professionally polished. They took off so much metal that the balance of each slat (and thus the whole stack) was knocked out of kinter. I had to fit a balance weight. A nasty business...

Having said that, I am not even sure that laser cutters will accept aluminium as a working material.

So, more research needed.

As regards Karen's Timing Corrector, this doesn't offer a completely 'free lunch'. If the intrinsic angular positioning of the slats requires appreciable correction, you can end up with a picture having jagged edges.

I shall have to study her (excellent) manual about whether the Corrector will work at 90-lines. She originally specified either 60 or 120, but there are in fact 128 'positions' available and I suspect the line count is actually determined by when the 're-setting' frame pulse comes round.

Steve O
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Re: A new 90-line Mirror Screw

Postby Steve Anderson » Sun Oct 06, 2024 12:42 pm

Panrock wrote:As regards Karen's Timing Corrector...I shall have to study her (excellent) manual about whether the Corrector will work at 90-lines. Steve O

Somehow I missed that publication, it would be an interesting read. Plus it may give clues as to how to modify the code for more or less lines and altering timings where and when needed. I can't recall if Karen ever published the corrector in the newsletter, on this forum or somewhere else.

As a concept it could possibly be applied to disc/drum/LED displays too. A TBC (Timebase Corrector) for mechanical and electronic displays.

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Re: A new 90-line Mirror Screw

Postby Panrock » Sun Oct 06, 2024 3:31 pm

I have the original 2016 464KB .pdf of the manual here. How would you like me to share it?

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Re: A new 90-line Mirror Screw

Postby Steve Anderson » Mon Oct 07, 2024 11:40 am

I would have said simply attach it to a short message here. But if that goes counter to established protocol, maybe attached to a PM? I'm thinking of copyright issues etc. (if any).

Plus Karen isn't with us anymore to seek permission to distribute the article and design within, or simply use it on a one-off basis.

I can only assume that the design and software (if any) hasn't been openly published anywhere. Which is unusual for Karen as most of her creations were openly available either here and/or via the newsletter, even if her coding was at times was somewhat cryptic...deliberate or not? No idea.

Steve A.

There's always a standard e-mail attacment too, I guess you may have my e-mail address from some time back, it hasn't changed in decades. Though I can send via PM if need be...sent already...
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Re: A new 90-line Mirror Screw

Postby Panrock » Mon Oct 07, 2024 7:11 pm

Thanks Steve. You now have a PM, with Karen's manual attached. I'm sure she'd have been happy for you to quote from it as necessary.

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Re: A new 90-line Mirror Screw

Postby Panrock » Tue Oct 08, 2024 4:11 am

I have found a substitute firm for the laser cutting and have received a positive initial response from them. I have now sent them the .dwg file of the slat (along with a picture of my previous mirror screw to stir their curiosity!).

I am inquiring as to whether they can do the polishing work too.

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Re: A new 90-line Mirror Screw

Postby Steve Anderson » Tue Oct 08, 2024 11:25 am

Hi Steve, only had a quick chance to look at Karen's work yesterday, but there's no code listing within. I also don't have the actual files she created, .ASM, .LST and so on, without which I would have to start over. Even with them they would need 'reverse engineering' and with Karens lack of commenting and cryptic assembly, that is a real challenge alone.

I'm not sure how to progress from here, I'll have a good look at the file you sent and send a follow-up...

In the interim I would hold-off any other related work or expenditure.

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Re: A new 90-line Mirror Screw

Postby Steve Anderson » Tue Oct 08, 2024 1:30 pm

The good news (hopefully) is that this is conceptually a lot simpler than I initially thought. It has nothing to do with motor speed, phasing control and all that tricky stuff, I assume that is all done within the Aurora. At least that's the way it looks to me at the moment.

That means it may be quicker and easier to simply create a MkIII version with the same I/O requirements for the mirror-screw (where required) and LED drivers. However, I'm not going to commit myself until I've done a few fag-packet sketches.

A selection of line quantity at the build stage via jumpers or dip-switches would be a useful addition. If any other additional 'feature' might be useful, now is the time to consider its addition.

Steve A.

Steve O, if there's any paperwork or a manual that came with the 'Aurora' that surely would be useful, though I may have some already...
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Re: A new 90-line Mirror Screw

Postby Panrock » Tue Oct 08, 2024 3:09 pm

As regards suitability of Karen's unit for 90-lines, I've just compared the .pdf I sent you with the paper copy of the Manual in my possession, and some relevant paragraphs that are present in the paper version are missing in the .pdf! The gist of the matter is I believe there should be no problem whatsoever in running the Timing Corrector at 90-lines.

On further inspection, I also see there are further big chunks missing from the .pdf I sent you, notably 'Firmware overview' and a troubleshooting guide. I'll have to see if I can scan all this and whether I still have OCR software somewhere... Alternatively they may have to be image files: .png and the like. I think it's unfair to expect any more from you Steve A, until this is resolved.

As for possible 'improvements', Karen and I discussed this, and all that came up was a possible feature to mask off the noticeable jagged edges of the picture that could arise when substantial different timing corrections are applied, line-by-line. However, I am happy to live with it as it is. The jagged edges provide me with a useful 'discipline' to encourage me to make a reasonably accurate mirror screw in the first place! To set up the slat angles, I use a modified snipe-nosed mole grip with facing 'chunks' removed from each jaw, which engage in the slat corners. The exact setting can be adjusted by turning the mole grip's adjuster screw. No doubt we'll meet this again later in the current project.

Steve A, I'll PM or email you the Aurora WC-01 Manual.

Steve O
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Re: A new 90-line Mirror Screw

Postby Steve Anderson » Tue Oct 08, 2024 4:33 pm

OK, got it...

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Re: A new 90-line Mirror Screw

Postby Steve Anderson » Tue Oct 08, 2024 4:46 pm

Panrock wrote:The jagged edges provide me with a useful 'discipline' to encourage me to make a reasonably accurate mirror screw in the first place!

Steve O

There's a chance to include an 'Offset Factor' for slat irregularities within this device. It somewhat depends on clock speeds and resolution. Each mirror-screw would need a unique offset file, a sort of 'fingerprint'. More thought required...

Also changing to a PIC18 micro from PIC16 allows over three times the processing speed plus larger internal RAM, an expanded instruction set, no time-wasting bank-switching instructions, and a possible reduction of pins from 40 to 28. Time will tell...

It would also be advantagous to replace the CA3306s with something more available today, and increase the bit-depth from 6 to 8 bits. Again, time will tell....

Also it's worth considering serial RAM, 8-pin devices instead of 20-plus pins. These and probably all the above available in DIP.

The only unknown is the fast 8-bit A-D...at the moment. For me this has always been the Achilles' heel of video conversion. Sure, they're aplenty for large-scale manufacuring, but for us mere mortals not easy to get, and Invariably in unfriendly packaging...I can cope with SOIC chips with a 1.27mm pin-spacing, just, but when it becomes 0.5mm, forget it!

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Re: A new 90-line Mirror Screw

Postby Steve Anderson » Wed Oct 09, 2024 2:14 pm

Panrock wrote:I've just compared the .pdf I sent you with the paper copy of the Manual in my possession, and some relevant paragraphs that are present in the paper version are missing in the .pdf! On further inspection, I also see there are further big chunks missing from the .pdf I sent you, notably 'Firmware overview' and a troubleshooting guide. I'll have to see if I can scan all this and whether I still have OCR software somewhere... Alternatively they may have to be image files: .png and the like. I think it's unfair to expect any more from you Steve A, until this is resolved.

Steve O

If you can scan said 'missing chunks', that's an improvement on nothing! If it's basically ledgible that should be OK, no real need for OCR. I realise this would be quite an effort, so don't go crazy on this.

Still searching for CA3306 replacements, they're few and far between, often hideously expensive (and three required), on long lead-times, require odd supply volts (e.g. 2.3V, an interfacing headache), or in unfriendly packaging, (TSSOP or similar)...or all of the above. A possible alternative to the CA3306 may be the AD7822 (in DIP and 8-bits, but shop around for the DIL/DIP version, though SOIC versions are a possibility), it depends on what sampling rate is required for the video coming out of the WC-01 in 90-line mode.

One thing I need clarifying, is the frame rate 25fps or 12.5fps at 90 lines? What, if any, are the line and/or frame sync arrangements?

A local supplier to me has 70 or so of the CA3306 in stock at US$4.00 each in one-off quantity. (Harris, Intersil or RCA, but no choice, you take what you get!). It's a possible short-term solution. But they're only 6-bits, not 8 (preferable). There was the CA3308 8-bit version, but I've never seen them listed anywhere. There's probably several thousand of them stacked up in the dark recesses of a warehouse somewhere totally forgotten!

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